1. Field of the Invention
This invention relates to combustion heating systems and control apparatus for such systems. More specifically, this invention relates to apparatus for constructing a furnace and its control system, to produce an induced draft furnace having increased efficiency.
2. Description of the Prior Art
Conventional gas-fired, natural draft furnace systems typically operate at a steady-state efficiency of about 75%. The seasonal average efficiency of such furnace systems is usually considerably lower, on the order of 60%. As the cost of gas and other fuels used for heating rises, and as such fuels grow scarcer, these levels of efficiency are considered less and less acceptable, and various ways of increasing furnace system efficiency are sought.
Several methods of increasing furnace efficiency are known in the prior art. For example, it is known that significant efficiency-reducing losses occur due to the escape of heat up the flue, vent, or exhaust stack during the portion of the furnace cycle when the burner is off. This heat is primarily heat taken from the burner heat exchanger following a burning cycle. One prior art solution to this form of heat loss is to provide dampers of various kinds which permit draft flow when required for the burning cycle, but serve to limit draft flow when the burner is not on. Examples of such dampers may be seen in the following U.S. Pat. Nos. 1,743,731; 1,773,585; 2,011,754; 2,218,930; 2,296,410; 4,017,024 and 4,108,369. As these patents show, a damper having the desired effect can be placed so as to limit exhaust draft flow out of the combustion chamber or input air flow into the combustion chamber.
A second form of efficiency-reducing loss in furnaces occurs due to inefficient burning as a result of improper air-fuel ratio. The prior art shows several methods for controlling fuel and/or air flow in order to maintain the air-fuel ratio as close as possible to the chemical ideal of stoichiometric burning, in which all fuel and oxygen would be completely combusted. Such prior art arrangements include U.S. Pat. No. 3,280,774, which shows an orifice plate of pre-selected cross-section and draft-limiting characteristics combined with a draft blower fan, and U.S. Pat. No. 2,296,410, which shows an apparatus for mechanically linking a modulating fuel regulator to a draft damper, to regulate the air supply in relation to the fuel supply.
A third form of efficiency-reducing loss in furnaces occurs due to the heat exchange process. Because it is impossible to transfer all the heat from the combustion chamber to the circulated air, water or other heat delivery medium, a certain amount of unabsorbed heat passes out of the heat exchanger and up the exhaust stack. One known way of reducing this type of loss is to derate the furnace, i.e., operate it at a lower firing rate. This permits a higher percentage of the heat produced by combustion to be absorbed in the heat exchanger. An example of a prior art patent disclosing a burner using derating is U.S. Pat. No. 3,869,243.
There are, however, certain disadvantages which may accompany a reduced firing rate. In particular, the following may arise: (1) slower response time in reaching the thermostatically selected room temperature; (2) possible inability to achieve the selected temperature; (3) increased condensation on the inside walls of the furnace chamber, or the interiors of tubing, valves, etc., associated with the furnace, leaading to more rapid corrosion, rusting or other deterioration of such parts; and (4) mismatching of fuel and air ratios, often leading to high excess air conditions at firing rates below the design maximum.